Two types of radiometer were used to measure the radiant energy produced by high energy helical xenon flashlamps operating in the pulse mode. Metal cylinders plated with platinum black are accurate to 2%, but can be used only at low incident energy and power. Graphite slugs are less accurate because of uncertainties in the reflectivity of graphite, but are more durable, and have been used successfully at incident radiant energy of 126 J/cm2 and peak power of 50×103 W/cm2.